A parameter for determining the dialysis efficiency is the Kt/V value. In this context, K [ml/min] is the clearance (fictitious plasma volume which is freed from a particular substance per time unit, in other words a concentration of a substance) and is determined via the urea content before and after dialysis. Moreover, t [min] is the effective dialysis time, in other words the duration of therapy. V [ml] indicates the urea distribution volume, in other words 60% of the body mass (weight) in which the blood can circulate (body fluid content). Alternatively, the clearance K or the ratio of clearance to the distribution volume K/V also constitutes an indicator for the efficiency of an ongoing dialysis treatment.
In optics, the extinction constitutes a measure for the attenuation of radiation (e.g. visible light) after passing a medium. The extinction is dependent on the wavelength and is a logarithmic variable. The extinction of the optical sensor EOS applied against the extinction of a reference device EREF (see FIG. 1) up to a particular value approximately corresponds to a straight line and after that the characteristic line flattens out. This means that the characteristic line applied against a reference flattens out with an increasing concentration of light-absorbing substances. In FIG. 1, the extinction of the optical sensor EOS is applied against the extinction of a reference device EREF, the reference device including a wide linear range. In addition, the identity is evident here. Accordingly, the optical sensor is linear up to an extinction of about 0.2 and after that flattens out.
In the course of a dialysis therapy, an afore-mentioned behavior can equally be observed. This is especially applicable when a dialysate contains a plurality of light-absorbing substances, which is the case especially at the beginning of dialysis. FIG. 2 exemplifies the time course of the extinction both for the optical sensor and for the reference device. Here the two extinction characteristics are applied vis-à-vis the measuring period (in seconds). The difference between the two extinctions is relatively large especially at the beginning. In the further course, the curves are approaching each other, however, which is due to the decrease of light-absorbing substances. The optical sensor is provided in the linear range as soon as both characteristic lines lie on top of each other. In other words, especially at the beginning the outlet-side dialysate is strongly polluted, e.g. by urea or other uremic toxins. In this case, the extinction is very high and the sensor thus is provided in the non-linear range entailing very high uncertainty of measurement. With an increasing duration of dialysis, the pollution strongly decreases and the sensor enters the linear range in which the measuring values are in conformity with the measuring values of the reference device again.
In the daily routine of a hospital, a reference device having a wide linear measuring range usually is not available. Therefore, it cannot be stated whether or from when the optical sensor is provided within the linear range, and hence whether or from when the optical sensor behaves in such way as it should behave according to the reference device.